Presumptive gene A mutants of 0X174 blocked in single-stranded but not double-stranded DNA synthesis will have their gene assignment verified by two-demensional gel electrophoresis. ts mutants of gene A, known to be extremely C-terminal, will have their stage of replication block defined, as will ts mutants of gene C. Phage gene products that interact with C protein will be identified. Mutagenesis and UV-reactivation of phage ST-1 will be studied in E. coli K12 tif-1 which produces a heat-activable form of the host recA protein.